This application is a counterpart of Japanese Patent Application Ser. No. 326685/1999, filed Nov. 17, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to an electro-absorption modulator, and to a method of manufacturing an optical device from semiconductor materials.
An electro-absorption modulator is an opto-electronic device that modulates light intensity by modulating an electric field controlling absorption of the light. Electro-absorption modulators are used for various types of optical signal processing. In particular, the output of a semiconductor laser diode can be modulated more rapidly by an electro-absorption modulator than by modulation of the driving power of the laser diode itself. Electro-absorption modulators can be fabricated from semiconductor materials, enabling a modulator and laser to be integrated into the same semiconductor chip. Integrated laser-modulator chips with a distributed-feedback (DFB) laser are useful as transmitters in high-bandwidth fiber-optic communication systems.
Since the absorption of light generates heat, electro-absorption modulators are vulnerable to thermal damage. The optical input power level at which thermal damage occurs is referred to as the damage level. Thermal damage is discussed in xe2x80x9cReliability Study of InGaAlAs/InAlAs MQW Electro-absorption Modulator,xe2x80x9d a paper presented by H. Kamioka et al. at the Optoelectronics and Communications Conference (OECC) held at Makuhari Messe in Japan in July 1998, published in the OECC Technical Digest (pp. 452-453). This paper studies damage levels of different electro-absorption modulator structures and their reliability below the damage level, concluding that damage level and reliability are related to the ability of the structure to dissipate the heat generated by absorption. The high mesa structure, for example, is found to dissipate heat more effectively than the buried heterostructure and thus to have a higher damage level.
The buried heterostructure, using semi-insulating iron-doped indium phosphide (Fe-doped InP), has been employed in integrated laser-modulator chips designed for broadband communication applications, but in addition to its low damage level, this structure is comparatively difficult to fabricate, because of the difficulty of growing crystalline semi-insulating InP with sufficiently high electrical resistance. An alternative structure is the ridge waveguide structure described in xe2x80x9c76-km transmission over standard dispersion fiber at 10 Gbit/s using a high-power integrated laser modulator and a PIN receiver without any optical amplifierxe2x80x9d by D. Lesterlin et al. in a paper presented at the Wednesday afternoon poster session of the 1997 Optical Fiber Communication (OFC) conference, published in the OFC Technical Digest, pp. 199-200.
With the ridge waveguide structure, it is comparatively easy to achieve a broadband electro-absorption modulator, and the device can be fabricated with fewer crystal growth steps than are needed for a buried-heterostructure waveguide. The ridge structure is intermediate between the buried heterostructure and the high mesa structure, however, so its thermal damage level can be expected to be intermediate between the damage levels found in those two structures.
There is a general need to enable electro-absorption modulators to withstand higher levels of optical power, so that signals can be transmitted over greater distances in optical communication systems. In particular, electro-absorption modulators with a ridge waveguide structure need to have higher damage levels if the full benefits of the ridge structure are to be realized.
One object of the present invention is to provide electro-absorption modulators with higher damage levels.
The invented electro-absorption modulator has an optical waveguide with an optical input end and an optical output end. A first electrode is disposed above the optical waveguide. A second electrode is disposed below the optical waveguide. An electric field applied to the optical waveguide from the first and second electrodes modulates the absorption of light in the optical waveguide as the light travels from the input end to the output end.
According to one aspect of the invention, the electro-absorption modulator also has a heat sink running parallel to the optical waveguide on one or both sides. The heat sink cools the optical waveguide by conducting heat away, thereby preventing overheating of the optical waveguide as a whole. By conducting heat in the lengthwise direction of the optical waveguide, the heat sink also prevents localized hot spots from forming. These features improve the ability of the electro-absorption modulator to withstand high optical input levels.
The heat sink is preferably formed as a thin metal film, metals in general being good conductors of heat. The optical waveguide may have an inverted mesa structure, in which case the overhanging part of the optical waveguide can function as a spacer when the heat sink is formed.
According to another aspect of the invention, the electro-absorption modulator is structured so as to reduce optical absorption near the optical input end of the optical waveguide, where most of the optical absorption and heating occur in a conventional electro-absorption modulator. This reduction also improves the ability of the modulator to withstand high optical input levels.
One way to obtain the desired absorption reduction is to reduce the electric field applied to the optical input end of the waveguide. The first electrode typically includes a stripe running parallel to the optical waveguide, and a pad connected to an external power source. In this configuration, the electric field can be reduced by an electrical resistance that produces a voltage drop between the electrode pad and the end of the electrode stripe disposed above the optical input end of the waveguide.
For example, the stripe can be divided into two or more segments, which are coupled in series through interconnecting members offering a higher electrical resistance than the stripe itself. A voltage drop equal to the product of the higher resistance and the current flowing through the resistance is produced. The resistive interconnection is preferably offset to one side of the stripe, away from the optical waveguide, thereby protecting the optical waveguide from joule heating that occurs in the interconnection resistance. The offset distance can be selected to obtain the necessary degree of protection, provided the voltage drop remains within an acceptable limit. The interconnection may include a section having an electrical conductance that can be adjusted to obtain a desired voltage drop. In particular, the interconnection may include a thin-film resistor, which can be formed easily by standard semiconductor fabrication techniques such as vacuum evaporation, lift-off, and photolithography. The thin film may have a different composition from the stripe itself; the thin-film material or materials can be selected to obtain a desired electrical resistance.
Alternatively, light absorption can be reduced at the optical input end of the waveguide by providing the absorbing layer of the waveguide with a higher bandgap energy at the optical input end than at the optical output end. For example, the optical waveguide can be fabricated by selective crystal growth.
In another aspect of the invention, the pad of the first electrode is disposed near the optical input end of the optical waveguide, to reduce the amount of heat generated by current flow in the stripe of the first electrode, particularly near the optical input end of the waveguide. Thermal damage to the electro-absorption modulator is thereby prevented, in that the temperature of the optical input end of the optical waveguide is lowered. This configuration is conditional on the modulation frequency, in that the pad must be located so that the high-frequency component of the applied voltage propagates through the entire stripe, but this condition does not appear to present problems at the modulation frequencies used in communication systems at present.
In all of the above aspects of the invention, the optical waveguide is preferably a ridge waveguide. A ridge waveguide is easier to fabricate than a buried-heterostructure waveguide, and is better than a high-mesa waveguide at providing the conditions for single-mode wave propagation. An electro-absorption modulator with a ridge waveguide can also be adapted comparatively easily for broadband operation.
The invented electro-absorption modulator can be integrated with a semiconductor laser into a monolithic device.
The invention also provides a general method of fabricating a semiconductor optical device. The invented method includes the formation of an optical waveguide having an inverted-mesa channel, and the formation of a heat sink by use of the inverted-mesa channel as a spacer, so that the heat sink extends parallel to the optical waveguide. The heat sink is thereby formed without the need for a separate masking pattern, and is positioned to cool the optical waveguide by conducting heat away from the optical waveguide, without being so close to the waveguide as to interfere with wave propagation in the waveguide. The spacing between the optical waveguide and the heat sink can be adjusted by adjustment of the height of the channel.
The semiconductor optical device may have an upper cladding layer that is partly doped with an impurity element to form a contact layer. In this case, the heat sink and a thin impurity-source film can be formed in the same step, simplifying the fabrication process.